Diagnostic Accuracy of the Anti-Citrulline Antibody Assay for Rheumatoid Arthritis

Rheumatoid arthritis (RA) is the most common autoimmune rheumatic disease, but specific and practicable tests for its diagnosis are lacking. We evaluated the diagnostic accuracy of a new commercial ELISA in detecting anti-cyclic citrullinated peptide (CCP) antibodies for the diagnosis of RA. Anti-CCP antibodies were determined in 330 serum samples: 98 from RA patients and 232 from controls, including patients with connective tissue diseases, other rheumatic diseases, viral infections, Lyme disease, autoimmune thyroiditis, cancer, and monoclonal gammopathy, and sex- and age-matched healthy subjects. Intra- and interassay CVs were 5-13% and 9-17%, respectively. Rheumatoid factor (RF) was also assayed in every sample, and results were compared to anti-CCP for sensitivity and specificity. At a cutoff value of 50 units, sensitivity was 41% (confidence interval, 31-50%) and specificity was 97.8% (95-100%). Anti-CCP-positive RA patients had a mean antibody concentration of 1100 units (range, 57-3419 units), and anti-CCP-negative RA patients and controls had mean values of 7.6 and 6.8 units, respectively (range, 1-39 units). The area under the ROC curve was 0.71 (95% confidence interval, 0.63-0.78). RF had a higher sensitivity (62%) and a lower specificity (84%) than anti-CCP. When the two antibodies were used together, specificity was 99.6%. Anti-CCP antibody testing may be useful if performed concomitantly with RF assay to diagnose patients with suspected early RA.